In the first days of July 1918 some newspapers in Holland report that a Dutch trawler had picked up a crew of 5 men who had came down in the North Sea (near Buoy 4) "in a water plane of sizable proportions (and) of American nationality"
Airplane apparently had had mechanical problems.

Crew said to be American and men were taken on board of the fishing vessel. On request of crew the trawler then ran through the aircraft to let it sink.

The trawler later transferred the crew to a passing British warship.

Who has an idea of the identity of this presumably USN flying boat and crew?

I know of RAF Felixstowe losses and damages etc. over the North Sea on both 4 June and 4 July 1918, but the stories around these don't quite match with the newspaper stories as above.

Were there any USN a/c losses in early summer 1918, either thru enemy action or accidental, that resemble what was written in the newspapers?

Yes, I have that very informative book. The newspaper article is mentioned, however without further investigation as to possible identity of a/c or
crew.
(American?) crew wasn't interned and flying boat was sunk by trawler....

No answer I'm afraid, but checking my notes, the aircraft must have been either a US built Curtiss H-16 (a total of 158 was sent abroad) or a British aquired Curtiss/Felixstowe aquired flyingboat (nine taken on charge). I cannot think of any operational floatplane which could carry as many as five individuals.
The number of individuals rescued is also a slight problem since the H-16 had a crew of four and presumably did its British counterparts as well. Presumably there was no problem to increase the number to five individuals, but it is a bit odd.
We can also disregard all French flyingboats since all of those were smaller in size.

If I understand you correctly the incident must have happened in June (or possibly very early in July?) unless, the trawler's crew rushed into the newspaper the minute they arrived into harbour.

The most likley NAS should be Killingholme. The first US aircraft arrived on June 1, 1918 (22 H-16 onboard USS Jason) and if it was an American built aircraft I believe it has to be one of those.

However, since Killingholme at this stage still was a 'mixed' station, ie used by both RAF and US Navy, we cannot dismiss the thought that a US crew had 'borrowed' a RAF flyingboat when the incident happened, since there seems to be no US record of any incident like this.

As I said, no help but perhaps a way of narrowing down your search.

BTW I have no record of any RAF/Dutch incident on 4 July 1918. Can you please enlighten me?

As with the big RAF/German air combat on 4 June 1918, the story in my thread doesn't fit with regard to fate of the aircraft.

I checked online Dutch newspaper archives and the story first appeared on 3 July 1918 and was repeated with the very same words in several other papers on 4 July 1918.
First date reportedly when the fishing vessel ('Neutraal') had returned in her home port of IJmuiden, Holland and crew had given their account.
The incident with the 'water plane' most likely therefore not more than a week before up to 3 July, assuming that under the wartime conditions the fishermen stayed in home waters and wouldn't be out for a lengthy period.

It seems that the trawler 'Neutraal' was not only involved in this water plane mystery of June/July 1918 but also in the still not resolved ditched seaplane of 17 July 1919 with pilot Murphy..
This pilot was picked up by a passing Dutch freighter and put ashore in N.Germany, while the 'Neutraal' salvaged the engine of this also still unknown airplane.